l -theanine: A potential multifaceted natural bioactive amide as health supplement

The Protective Effects of L-Theanine against Epigallocatechin Gallate-Induced Acute Liver Injury in Mice

Epigallocatechin-3-gallate (EGCG) is a main bioactive constituent in green tea. Being a redox-active polyphenol, high-dose EGCG exhibits pro-oxidative activity and could cause liver injury. L-theanine is a unique non-protein amino acid in green tea and could provide liver-protective effects. The purpose of this study was to investigate the hepatoprotective effects of L-theanine on EGCG-induced liver injury and the underlying mechanisms. A total of 300 mg/kg L-theanine was administrated to ICR mice for 7 days. Then, the acute liver injury model was established through intragastric administration of 1000 mg/kg EGCG. Pretreatment with L-theanine significantly alleviated the oxidative stress and inflammatory response caused by high-dose EGCG through modulation of Nrf2 signaling and glutathione homeostasis. Furthermore, metabolomic results revealed that L-theanine protects mice from EGCG-induced liver injury mainly through the regulation of amino acid metabolism, especially tryptophan metabolism. These findings could provide valuable insights into the potential therapeutic applications of L-theanine and highlight the importance of the interactions between dietary components.

Comprehensive Review of Nutraceuticals against Cognitive Decline Associated with Alzheimer's Disease

Nowadays, nutraceuticals are being incorporated into functional foods or used as supplements with nonpharmacological approaches in the prevention and management of several illnesses, including age-related conditions and chronic neurodegenerative diseases. Nutraceuticals are apt for preventing and treating such disorders because of their nontoxic, non-habit-forming, and efficient bioactivities for promoting neurological well-being due to their ability to influence cellular processes such as neurogenesis, synaptogenesis, synaptic transmission, neuro-inflammation, oxidative stress, cell death modulation, and neuronal survival. The capacity of nutraceuticals to modify all of these processes reveals the potential to develop food-based strategies to aid brain development and enhance brain function, prevent and ameliorate neurodegeneration, and possibly reverse the cognitive impairment observed in Alzheimer's disease, the most predominant form of dementia in the elderly. The current review summarizes the experimental evidence of the neuroprotective capacity of nutraceuticals against Alzheimer's disease, describing their mechanisms of action and the in vitro and in vivo models applied to evaluate their neuroprotective potential.

Effects of Vegetal Extracts and Metabolites against Oxidative Stress and Associated Diseases: Studies in Caenorhabditis elegans

Oxidative stress is a natural physiological process where the levels of oxidants, such as reactive oxygen species (ROS) and nitrogen (RNS), exceed the strategy of antioxidant defenses, culminating in the interruption of redox signaling and control. Oxidative stress is associated with multiple pathologies, including premature aging, neurodegenerative diseases, obesity, diabetes, atherosclerosis, and arthritis. It is not yet clear whether oxidative stress is the cause or consequence of these diseases; however, it has been shown that using compounds with antioxidant properties, particularly compounds of natural origin, could prevent or slow down the progress of different pathologies. Within this context, the Caenorhabditis elegans (C. elegans) model has served to study the effect of different metabolites and natural compounds, which has helped to decipher molecular targets and the effect of these compounds on premature aging and some diseases such as neurodegenerative diseases and dyslipidemia. This article lists the studies carried out on C. elegans in which metabolites and natural extracts have been tested against oxidative stress and the pathologies associated with providing an overview of the discoveries in the redox area made with this nematode.

The role of dietary antioxidants in type 2 diabetes and neurodegenerative disorders: An assessment of the benefit profile

Healthy diet is vital to cellular health. The human body succumbs to numerous diseases which afflict severe economic and psychological burdens on the patient and family. Oxidative stress is a possible crucial regulator of various pathologies, including type 2 diabetes and neurodegenerative diseases. It generates reactive oxygen species (ROS) that trigger the dysregulation of essential cellular functions, ultimately affecting cellular health and homeostasis. However, lower levels of ROS can be advantageous and are implicated in a variety of signaling pathways. Due to this dichotomy, the terms oxidative "eustress," which refers to a good oxidative event, and "distress," which can be hazardous, have developed. ROS affects multiple signaling pathways, leading to compromised insulin secretion, insulin resistance, and β-cell dysfunction in diabetes. ROS is also associated with increased mitochondrial dysfunction and neuroinflammation, aggravating neurodegenerative conditions in the body, particularly with age. Treatment includes drugs/therapies often associated with dependence, side effects including non-selectivity, and possible toxicity, particularly in the long run. It is imperative to explore alternative medicines as an adjunct therapy, utilizing natural remedies/resources to avoid all the possible harms. Antioxidants are vital components of our body that fight disease by reducing oxidative stress or nullifying the excess toxic free radicals produced under various pathological conditions. In this review, we focus on the antioxidant effects of components of dietary foods such as tea, coffee, wine, oils, and honey and the role and mechanism of action of these antioxidants in alleviating type 2 diabetes and neurodegenerative disorders. We aim to provide information about possible alternatives to drug treatments used alone or combined to reduce drug intake and encourage the consumption of natural ingredients at doses adequate to promote health and combat pathologies while reducing unwanted risks and side effects.

Dissecting the molecular mechanisms underlying the antidepressant activities of herbal medicines through the comprehensive review of the recent literatures

Depression is clinically defined as a mood disorder with persistent feeling of sadness, despair, fatigue, and loss of interest. The pathophysiology of depression is tightly regulated by the biosynthesis, transport and signaling of neurotransmitters [e.g., serotonin, norepinephrine, dopamine, or γ-aminobutyric acid (GABA)] in the central nervous system. The existing antidepressant drugs mainly target the dysfunctions of various neurotransmitters, while the efficacy of antidepressant therapeutics is undermined by different adverse side-effects. The present review aimed to dissect the molecular mechanisms underlying the antidepressant activities of herbal medicines toward the development of effective and safe antidepressant drugs. Our strategy involved comprehensive review and network pharmacology analysis for the active compounds and associated target proteins. As results, 45 different antidepressant herbal medicines were identified from various in vivo and in vitro studies. The antidepressant mechanisms might involve multiple signaling pathways that regulate neurotransmitters, neurogenesis, anti-inflammation, antioxidation, endocrine, and microbiota. Importantly, herbal medicines could modulate broader spectrum of the cellular pathways and processes to attenuate depression and avoid the side-effects of synthetic antidepressant drugs. The present review not only recognized the antidepressant potential of herbal medicines but also provided molecular insights for the development of novel antidepressant drugs.

The Effect of In Vitro Digestion on Matcha Tea (Camellia sinensis) Active Components and Antioxidant Activity

This study investigates the effects of in vitro digestion on the antioxidant activity and release of phenolics, xanthine alkaloids, and L-theanine contents of matcha. It establishes digestibility values between 61.2–65.8%. Considering native matcha, the rutin content (303–479 µg/g) reached higher values than catechin (10.2–23.1 µg/g). Chlorogenic acid (2090–2460 µg/g) was determined as predominant. Rutin, quercetin, ferulic, ellagic, and caffeic acid were the least-released phenolics, and their remaining residues reached 76–84%. Protocatechuic, hydroxybenzoic acid, epigallocatechin, and epigallocatechin-3-gallate were the best-released phenolics, with the remaining residues under 1%. Caffeine, L-theanine, and theobromine contents in native matcha reached 16.1, 9.85, and 0.27 mg/g, respectively. Only caffeine (3.66–5.26 mg/g) and L-theanine (0.09–0.15 mg/g) were monitored in the undigested residue, representing 13 and 0.1% of the remaining part, respectively. A chemiluminescence assay showed that water-soluble antioxidants showed significant antioxidant activity in native matcha, while lipid-soluble compounds showed higher antioxidant activity in the undigested samples. Cinnamic and neochlorogenic acids were determined as the main contributors to the ACW values in the undigested matcha, epicatechin, and quercetin in the ACL fraction. The application of the digestion process reduced the antioxidant activity by more than 94%. SEM has proved specific digestion patterns of in vitro digestibility of matcha.

L-Theanine: A Unique Functional Amino Acid in Tea (Camellia sinensis L.) With Multiple Health Benefits and Food Applications

Tea (Camellia sinensis L.) is a very popular health drink and has attracted increasing attention in recent years due to its various bioactive substances. Among them, L-theanine, a unique free amino acid, is one of the most important substances in tea and endows tea with a special flavor. Moreover, L-theanine is also a bioactive compound with plenty of health benefits, including antioxidant, anti-inflammatory, neuroprotective, anticancer, metabolic regulatory, cardiovascular protective, liver and kidney protective, immune regulatory, and anti-obesity effects. Due to the unique characteristics and beneficial functions, L-theanine has potential applications in the development of functional foods. This review summarized the influencing factors of L-theanine content in teas, the main health benefits and related molecular mechanisms of L-theanine, and its applications in food, understanding of which can provide updated information for the further research of L-theanine.

Functional roles of taurine, L-theanine, L-citrulline, and betaine during heat stress in poultry

Heat stress (HS) is an important environmental stress factor affecting poultry production on a global scale. With the rise in ambient temperature and increasing effects of global warming, it becomes pertinent to understand the effects of HS on poultry production and the strategies that can be adopted to mitigate its detrimental impacts on the performance, health, welfare, immunity, and survival of birds. Amino acids (AAs) have been increasingly adopted as nutritional modifiers in animals to ameliorate the adverse effects of HS. They are essential for protein synthesis, growth, maintenance, reproduction, immunity, stress response, and whole-body homeostasis. However, HS tends to adversely affect the availability, transport, absorption, and utilization of these AAs. Studies have investigated the provision of these AAs to poultry during HS conditions, and variable findings have been reported. Taurine, L-theanine, and L-citrulline are non-essential amino acids that are increasingly gaining attention as nutritional supplements in HS animals. Similarly, betaine is an amino acid derivative that possesses favorable biological properties which contributes to its role as a functional additive during HS. Of particular note, taurine is negligible in plants, while betaine, L-theanine, and L-citrulline can be found in selected plants. These nutrients are barely found in feed ingredients, but their supply has been shown to elicit important physiological roles including anti-stress effects, anti-oxidative, anti-inflammatory, gut promoting, and immunomodulatory functions. The present review provides information on the use of these nutritionally and physiologically beneficial nutrients as functional additives to poultry diets during HS conditions. Presently, although several studies have reported on the positive effects of these additives in human and murine studies, however, there is limited information regarding their utilization during heat stress in poultry nutrition. Therefore, this review aims to expound on the functional properties of these nutrients, their potentials for HS alleviation, and to stimulate further researches on their biological roles in poultry nutrition.

Effect of Alpha-S1-Casein Tryptic Hydrolysate and L-Theanine on Poor Sleep Quality: A Double Blind, Randomized Placebo-Controlled Crossover Trial

This randomized, placebo-controlled, crossover and double-blind study investigates the effects of RLX2™ containing alpha-s1-casein tryptic hydrolysate and L-theanine in working adults affected by poor sleep quality. The supplement or placebo was randomly and blindly assigned to 39 subjects for four weeks and the changes in the subjective sleep assessment via the Pittsburgh Sleep Quality Index (PSQI), heart rate, blood pressure, salivary cortisol by high-performance liquid chromatography method and alpha power of awake electroencephalogram (EEG) were studied. The data were analyzed in two ways, by crossover and crossover summed up. The latter depicted that RLX2™ improved PSQI total score, sleep latency, sleep duration, sleep habitual efficiency, daytime dysfunction, and increased total and frontal alpha power significantly (p < 0.05). The supplement prolonged the total sleeping time by 45 min in the supplement receiving group compared to the placebo group (p < 0.001). However, only sleep duration and sleep habitual efficiency showed a profound effect in both analyses (p < 0.05). In conclusion, being given its beneficial effects without notable adverse events, it would be advantageous to use these nutraceutical ingredients to promote better sleep quality. Further studies with a larger number participants are warranted to support these findings.

Quantitative trait loci mapping for free amino acid content using an albino population and SNP markers provides insight into the genetic improvement of tea plants

Free amino acids are one of the main chemical components in tea, and they contribute to the pleasant flavor, function, and quality of tea, notably the level of theanine. Here, a high-density genetic map was constructed to characterize quantitative trait loci (QTLs) for free amino acid content. A total of 2688 polymorphic SNP markers were obtained using genotyping-by-sequencing (GBS) based on 198 individuals derived from a pseudotestcross population of "Longjing 43" × "Baijiguan", which are elite and albino tea cultivars, respectively. The 1846.32 cM high-density map with an average interval of 0.69 cM was successfully divided into 15 linkage groups (LGs) ranging from 93.41 cM to 171.28 cM. Furthermore, a total of 4 QTLs related to free amino acid content (theanine, glutamate, glutamine, aspartic acid and arginine) identified over two years were mapped to LG03, LG06, LG11 and LG14. The phenotypic variation explained by these QTLs ranged from 11.8% to 23.7%, with an LOD score from 3.56 to 7.7. Furthermore, several important amino acid metabolic pathways were enriched based on the upregulated differentially expressed genes (DEGs) among the offspring. These results will be essential for fine mapping genes involved in amino acid pathways and diversity, thereby providing a promising avenue for the genetic improvement of tea plants.

A Randomized, Triple-Blind, Placebo-Controlled, Crossover Study to Investigate the Efficacy of a Single Dose of AlphaWave® L-Theanine on Stress in a Healthy Adult Population

Introduction

Stress is a complex life occurrence essential for survival and goal achievement but can be damaging in excess. Because of the high prevalence of stress in North America, a safe supplement that effectively reduces stress is in demand. The objective of this study was to investigate the efficacy and safety of AlphaWave^®l-Theanine on whole-scalp and frontal alpha power, midline theta power, and salivary cortisol in healthy, moderately stressed adults.

Methods

This was a randomized, triple-blind, placebo-controlled, crossover study that consisted of two study periods with a 7-day washout. A single dose of AlphaWave^®l-Theanine (200 mg) or placebo was administered. To induce stress, a mental arithmetic test (MAT) was administered before and after the dose. Electroencephalogram, salivary cortisol, blood pressure, heart rate, self-reported stress, adverse events, clinical chemistry, and hematology were assessed to evaluate efficacy and safety.

Results

Increases in heart rate, blood pressure, and self-reported stress and state anxiety indicated that participants experienced stress during the MAT. AlphaWave^®l-Theanine led to a greater increase in frontal region and whole-scalp alpha power 3 h post-dose compared to placebo (p ≤ 0.050). Within groups, there were increases in alpha power, at 3 h with AlphaWave^®l-Theanine, over the whole recording and during the eyes-open portions (p ≤ 0.048) of the alpha task. The changes in alpha wave activity are supported by greater decreases in salivary cortisol 1 h post-dose (p < 0.001) with AlphaWave^®l-Theanine compared to placebo.

Conclusion

This study was conducted during the SARS-CoV-2 pandemic, which has had a rapid and significant effect on both physical and mental health around the world. A single dose of AlphaWave^®l-Theanine significantly increased frontal region alpha power compared to placebo in response to an acute stress challenge. These changes are indicative of relaxation in the brain and suggest a calming response. AlphaWave^®l-Theanine was found to be safe and well tolerated by participants.

Trial Registration

ClinicalTrials.gov identifier NCT04706494.

Stress is a complex part of life that is essential for survival and achieving goals. Too much stress, however, can be damaging. There is a high prevalence of stress in North America, creating a demand for a safe and effective supplement to reduce it. This study investigated the effectiveness and safety of AlphaWave^®l-Theanine on brainwaves and salivary cortisol in healthy, moderately stressed adults facing an acute stressor. This was a randomized, triple-blind, placebo-controlled, crossover study that consisted of two study periods with a 7-day washout. A single dose of 200 mg of AlphaWave^®l-Theanine or placebo was administered before and after a mental arithmetic test to elicit acute stress. Electroencephalogram, salivary cortisol, blood pressure, heart rate, self-reported stress, and safety were assessed to evaluate efficacy and safety. This study was conducted during the coronavirus pandemic, which has had a rapid and significant effect on both physical and mental health around the world. A single dose of AlphaWave^®l-Theanine had significant positive effects on brainwaves, salivary cortisol, and self-reported state anxiety compared to the placebo in response to an acute stress challenge. These changes are indicative of relaxation in the brain and suggest a calming response in a moderately stressed but otherwise healthy population. AlphaWave^®l-Theanine was found to be safe and well tolerated by participants.

Development and Application of a Fast Gas Chromatographic Method Offer New Insights into l-theanine Production Regulation in Camellia sinensis L

Tea is the most consumed beverage worldwide, and l-theanine in tea leaves significantly affects their flavor and market quality. We have developed and validated a fast and reliable gas chromatographic method with flame ionization detection (GC-FID) to quantify l-theanine after its extraction from Camellia sinensis (tea plant) and derivatization. The procedure was completed in 40 min, from extraction to chromatographic analysis, with a recovery rate of more than 93% and allowing a high sample throughput. The GC-FID intraday precision was within 0.57-2.28%, while the interday precision ranged from 1.57 to 13.48%. The intraday accuracy ranged from -6.84 to 5.26%, while the interday accuracy ranged from -1.08 to 3.12%. The limit of detection was 2.28 μg/mL, and the limit of quantification was 6.47 μg/mL. The GC-FID method was validated by high-performance liquid chromatography with UV detection (HPLC-UV) and was used to investigate the biosynthesis and regulation of l-theanine in tea plants. We found that plants fed with ethylamine significantly increased l-theanine concentrations in roots, while exogenous supplementation of glutamic acid, carbamide, and glutamine did not significantly affect the l-theanine level in roots. Our results also indicated that roots were not indispensable for the biosynthesis of l-theanine, which was detected in undifferentiated embryonic calluses in concentrations (g/100 g dry weight) as high as in leaves of whole plants (1.67 and 1.57%, respectively) and without any exogenous theanine precursor supplementation.

Theanine and cancer: A systematic review of the literature

A growing literature indicates several health benefits of theanine, a major nonprotein derivative amino acid special to tea, and a nonedible mushroom. This study aimed to systematically review the scientific evidence regarding the anticarcinogen and anticancer effects of natural theanine. A systematic search for the relevant articles published until January 2021 on MEDLINE, Scopus, and Web of Knowledge was conducted. Out of 377 initial records, 14 in vitro, ex vivo, and in vivo studies met our inclusion criteria. Most of the included in vitro and ex vivo studies reported beneficial effects of theanine on the proliferation, apoptosis, metastasis, migration, and invasion in various cancer cell lines. The in vivo studies also supported the potential impacts of theanine on cancer incidence or progression. Theanine exerted its anticancer function by inhibiting EGFR, VEGFR, Met, and Akt/mTOR, JAK2/STAT3, and ERK/NFκB pathways, as well as activating the intrinsic apoptosis pathway and caspase-independent programmed cell death. In conclusion, the results indicated moderate apoptotic, antimetastatic, antimigration, and anti-invasion effects, along with the mild antiproliferative influence of theanine on cancer. Further studies are necessary to ascertain the effectiveness of theanine on the prevention and suppression of cancer and shed light upon the attributable mechanisms in the in vivo condition.

Magnetic Cross-Linked Enzyme Aggregates of a Transpeptidase-Specialized Variant (N450D) of Bacillus licheniformis γ-Glutamyl Transpeptidase: An Efficient and Stable Biocatalyst for l-Theanine Synthesis

γ-Glutamyl transpeptidase (GGT) catalyzes the transfer of glutathione’s γ-glutamyl group and related γ-glutamyl amides to water, amino acids or peptides, and utilizes a conserved Thr residue to process its own polypeptide chain into a large and a small subunit that then assemble to produce a catalytically competent enzyme. In this study, the magnetic cross-linked enzyme aggregates (mCLEAs) of a transpeptidase-specialized variant (N450D) of Bacillus licheniformis GGT were successfully prepared with optimized process parameters viz.1.25:1 (v/v) of isopropanol to N450D (0.3 mg/mL) ratio/0.02:1 (w/w) of enzyme to 3-aminopropyl triethoxysilane (APTES)-coated magnetic nanoparticle ratio/20 mM of glutaraldehyde. The prepared magnetic nanoparticles and immobilized enzyme (N450D-mCLEAs) were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscope integrated with energy dispersive X-ray spectroscopy (FESEM/EDS), and superparamagnetic analysis. As compared with free enzyme, N450D-mCLEAs displayed significantly higher heat resistance at temperatures of 55 and 60 °C, and had a greater stability over a storage period of one month. The immobilized enzyme could also be reused for 10 consecutive biocatalytic cycles with no significant reduction in the percent yield of l-theanine. Conclusively, this immobilization strategy surely provides a meaningful glance of developing N450D-mediated biocatalysis for the production of physiologically important γ-glutamyl compounds.

Nutrition and aroma challenges of green tea product as affected by emerging superfine grinding and traditional extraction

Green tea, superfine green tea powder (SGTP), and tea extract were prepared to determine their chemical components and antioxidant activity. The nutrition and aroma challenges of green tea during traditional extract technique and superfine grinding were profiled in this study. The traditional extract technique took advantage over superfine grinding in L-theanine and glutamic acid (Glu) preservation, but not in other 16 essential amino acids. SGTP preserved a maximum of elements from green tea, whereas tea extract greatly changed the elements ratio. Tea extract contained higher VB2 and VC contents and doubled the tea polyphenol (TP) content in comparison with green tea and SGTP. Additionally, tea extract contained more favorable aroma compounds and maintained stronger antioxidant activity in comparison with green tea and SGTP. This study profiled an important basis for the comprehensive utilization of green tea resources by consumers and manufacturers.

Production of L-Theanine Using Escherichia coli Whole-Cell Overexpressing γ-Glutamylmethylamide Synthetase with Bakers Yeast

L-Theanine, found in green tea leaves has been shown to positively affect immunity and relaxation in humans. There have been many attempts to produce L-theanine through enzymatic synthesis to overcome the limitations of traditional methods. Among the many genes coding for enzymes in the L-theanine biosynthesis, glutamylmethylamide synthetase (GMAS) exhibits the greatest possibility of producing large amounts of production. Thus, GMAS from Methylovorus mays No. 9 was overexpressed in several strains including vectors with different copy numbers. BW25113(DE3) cells containing the pET24ma::gmas was selected for strains. The optimal temperature, pH, and metal ion concentration were 50°C, 7, and 5 mM MnCl2, respectively. Additionally, ATP was found to be an important factor for producing high concentration of L-theanine so several strains were tested during the reaction for ATP regeneration. Bakers yeast was found to decrease the demand for ATP most effectively. Addition of potassium phosphate source was demonstrated by producing 4-fold higher L-theanine. To enhance the conversion yield, GMAS was additionally overexpressed in the system. A maximum of 198 mM L-theanine was produced with 16.5 mmol/l/h productivity. The whole-cell reaction involving GMAS has greatest potential for scale-up production of L-theanine.

Secretion of Bacillus amyloliquefaciens γ-Glutamyltranspeptidase from Bacillus subtilis and Its Application in Enzymatic Synthesis of l-Theanine

In this study, the gene of γ-glutamyltranspeptidase (GGT) from Bacillus amyloliquefaciens (BaGGT) controlled by the P_lac promoter was cloned into Bacillus subtilis to construct two recombinant vectors with either one or two signal peptides to drive extracellular secretion. After optimization, 90 ± 0.2 mg/L BaGGT was obtained when the inducing conditions were 24 h and 80 μM (IPTG). The properties of BaGGT were measured, showing that the optimal reaction conditions were 40 °C and pH 9.0 with 55.0 ± 0.5 U/mg enzymatic activity. K_m and V_max were 0.214 mM and 88.13 μmol/min/mg. BaGGT could be stored for 72 h with 90% of the initial activity at 40 °C and retained more than 50% of the initial activity after being maintained at different pH values for 24 h. Finally, enzymatic synthesis of l-theanine was performed with the optimal conditions: 20 mM l-Gln, 100 mM ethylamine HCl, 0.5 U/mL BaGGT, incubated at 40 °C for 6 h, 200 rpm.

Psychotropic effects of L-theanine and its clinical properties: From the management of anxiety and stress to a potential use in schizophrenia

Anxiety disorders are highly prevalent in modern societies, and are ranked the sixth most important contributor of non-fatal negative health outcomes. _L-theanine is an amino acid naturally found in green tea (Camellia sinensis) and some other plant extracts, and recent clinical studies have proposed promising adjuvant effects of _L-theanine for the negative impact of anxiety and psychological stress on health. In this integrative narrative review, we aimed to appraise and further discuss the effects of _L-theanine administration on anxiety disorders and psychological stress. Published data suggests that _L-theanine administered at daily doses ranging from 200 to 400 mg for up to 8 weeks are safe and induce anxiolytic and anti-stress effects in acute and chronic conditions. _L-theanine at doses lower and higher than these may also show promising therapeutic potential; however, a more thorough investigation through randomized double-blind placebo-controlled crossover clinical trials are necessary to elucidate its effects for longer periods, providing further insights for meta-analyses and the development of recommendation guidelines. Additionally, animal studies investigating a higher dosage, its combination with other pharmacological compounds and associated metabolic comorbidities are recommended, as cases of hepatotoxicity associated with the consumption of green tea extract have been reported.

Serum Ethylamine Levels as an Indicator of l-Theanine Consumption and the Risk of Type 2 Diabetes in a General Japanese Population: The Hisayama Study

OBJECTIVE

This study investigated the association between serum ethylamine levels as an indicator of l-theanine consumption and the development of type 2 diabetes in a Japanese community.

RESEARCH DESIGN AND METHODS

A total of 2,253 community-dwelling Japanese individuals aged 40-79 years without diabetes were monitored for 7 years. Serum ethylamine levels were divided into quartiles: ≤0.86, 0.87-2.10, 2.11-5.28, and ≥5.29 ng/mL. Kinetic analysis of serum ethylamine concentrations was performed after ingestion of l-theanine-rich green tea products containing 8 mg of l-theanine by 12 healthy volunteers.

RESULTS

During follow-up, 282 subjects developed type 2 diabetes. The age- and sex-adjusted cumulative incidence of type 2 diabetes decreased significantly with elevating levels of serum ethylamine (P for trend = 0.04). This association remained unchanged after adjusting for potential confounding factors. The multivariable-adjusted hazard ratio (HR) for type 2 diabetes was significantly lower in the fourth quartile of serum ethylamine than in the first quartile (HR 0.69, 95% CI 0.49-0.98). This trend of decrease in diabetic risk across serum ethylamine levels was more prominent in middle-aged subjects and in subjects with prediabetes, obesity, or insulin resistance. Kinetic analysis estimated that the minimum concentration at the steady state was >5.90 ng/mL in the case of twice-daily ingestion with an interval of 12 h.

CONCLUSIONS

Higher serum ethylamine was significantly associated with lower risk of the development of type 2 diabetes in a general Japanese population. The measurement of serum ethylamine concentration would be a useful biomarker for the objective estimation of l-theanine consumption.